[{user:12,process_name:"a"},{user:12,process_name:"b"},{user:34,process_name:"c"},{user:36,process_name:"a"},{user:36,process_name:"d"},{user:12,process_name:"p"}]

db.collection.aggregate([{$group:{_id:"$user",users:{$addToSet:"$process_name"},total:{$sum:1}}},{$addFields:{size_ab:{$size:{$filter:{input:"$users",cond:{$in:["$$this",["a","b"]]}}}}}},{$project:{_id:0,user:"$_id",percentage:{$divide:["$size_ab","$total"]}}}])